Add a novision build tag so the server can be built and tested off-device

[yuzi-co]

common/kvm_vision.go links against libkvm through cgo, so any package that reaches it can only be built with the cross-compiler and the device libraries present. That is most of the server, which means none of it can be type checked or tested on a workstation or in CI.

This adds a novision tag that swaps that one file for a pure Go stub with the same surface:

go vet -tags novision ./...
go test -tags novision ./...
go build -tags novision ./...

The device build is untouched. Without the tag the real bindings are used exactly as before, and common/kvm_vision.go only gains a //go:build !novision line.

I opened this first because several other fixes I am proposing come with tests, and without something like this there is no way to run them anywhere but on hardware.

One thing worth knowing

With the tag in place the existing suite runs, and it is not quite green under load. service/controlmode and service/picoclaw have tests with one-second timing budgets that fail when the whole suite runs in parallel on a slow filesystem, and pass consistently when run alone:

go test -tags novision -count=1 ./service/controlmode/   ok (3/3 runs)
go test -tags novision -count=1 ./service/picoclaw/      ok (2/2 runs)
go test -tags novision -count=1 -p 1 ./...               intermittent

Those are pre-existing and nothing to do with this change, but if you wire this into CI you will meet them. -p 1 makes it reliable in my testing.

Verified: go build, go vet, and GOOS=linux GOARCH=riscv64 go build all clean with and without the tag.